Not all infectious diseases are transmitted by bacteria or viruses. Some neurological diseases, such as Creutzfeldt-Jaakob disease (CJD) or mad cow disease are in fact caused by agents called Prions, which are of similar size to viruses but are made up of only protein[1]

Prions have these characteristics:

1.) "The transmissible agent consists of aggregated forms of a specific protein" [2].

2.) These protein aggregates cannot be degraded by the agents that degrade most other proteins [3].

3.) "The protein is largely or completely derived from a cellular protein called PrP, that is normally present in the brain" [4].

So an aggregated form of a protein (that is already present in the brain) is the infectious agent in prion diseases [5].

The pathalogic mechanism of prions is often associated with structural change. Exogenous prions cause the endogenous host proteins to undergo a structural change, rendering them functionless or harmful.